Biological process for removing phoshporus involving a membrane filter

ABSTRACT

A waste water treatment process for biologically removing phosphates incorporates a membrane filter. The process includes three zones, an anaerobic zone, an anoxic zone and an aerobic zone containing an anaerobic, anoxic and aerobic mixed liquor. Water to be treated flows first into the anaerobic zone. Anaerobic mixed liquor flows to the anoxic zone. Anoxic mixed liquor flows both back to the anaerobic zone and to the aerobic zone. The aerobic mixed liquor flows to the anoxic zone and also contacts the feed side of a membrane filter. The membrane filter treats the aerobic mixed liquor to produce a treated effluent lean in phosphorous, nitrogen, BOD OR COD, suspended solids and organisms at a permeate side of the membrane filter and a liquid rich in rejected solids and organisms. Some or all of the material rejected by the membrane filter is removed from the process either directly or by returning the material rejected by the membrane filter to the anoxic or aerobic zones and wasting aerobic sludge. In a first optional side stream process, phosphorous is precipitated from a liquid lean in solids extracted from the anaerobic mixed liquor. In a second optional side stream process, anaerobic mixed liquor is treated to form insoluble phosphates which are removed in a hydrocyclone.

[0001] This is a division of U.S. patent application Ser. No. 09/646,115filed Sep. 27, 2000 which is a National Stage of PCT application numberPCT/CA00/00854 filed on Jul. 19, 2000. The entire disclosure both ofthese of applications is incorporated herein by this reference.

FIELD OF THE INVENTION

[0002] The present invention relates to waste water treatment, and moreparticularly to a process for removing phosphorous from waste waterinvolving biological processes and a membrane filter.

BACKGROUND OF THE INVENTION

[0003] BOD, nitrates and phosphates released into the environment causeeutrophication and algae blooming resulting in serious pollution andhealth problems. Waste water treatment processes attempt to remove BOD,nitrates and phosphates to produce an acceptable effluent.

[0004] Conventional processes for removing phosphates from waste waterinclude chemical precipitation and biological methods. In chemicalprecipitation methods, soluble salts, such as ferrous/ferric chloride oraluminum sulphate, are added to the waste water to form insolublephosphate metal salts. The waste water, however, contains many differentions which create undesirable side reactions with the precipitants. As aresult, and particularly where very low effluent total phosphorus levelsare required, these processes may require the addition of 5-6 times thestoichiometric amount of chemicals required to remove the phosphates.Accordingly, these processes result in high chemical costs, high sludgeproduction, and a high level of metallic impurities in the sludge.

[0005] In contrast, biological methods use microorganisms to digest thephosphates. For example, U.S. Pat. No. 4,867,883 discusses a processwhich attempts to create conditions which encourage the selection andgrowth of Bio-P organisms, a strain of bacteria which have the abilityto uptake phosphorus in excess of the amount normally needed for cellgrowth. The amount of phosphorus removal that can be achieved isdirectly proportional to the amount of Bio-P organisms in the system.Generally, the process consists of an anaerobic zone, an anoxic zone, anaerobic zone, a clarifier, and a variety of recycles to interconnect thevarious zones. In a preferred embodiment of the process, there is adenitrified recycle from the anoxic zone to the anaerobic zone, anitrified recycle from the aerobic zone to the anoxic zone, and anactivated sludge recycle from the clarifier to the anoxic zone. In theanaerobic zone, there is BOD assimilation and phosphorus release.Subsequently, in the anoxic and aerobic zones, there is phosphorusuptake. In the clarifier, sludge containing phosphates settles out ofthe effluent. In some cases, sand filters are employed to try to furtherreduce the amount of phosphates in the effluent.

[0006] One problem with the U.S. Pat. No. '883 process is that there canbe a build up of phosphates in the system. At the end of the process, aportion of the recycled activated sludge is wasted and is subsequentlytreated, typically by aerobic or anaerobic digestion processes. Thisresults in a release of phosphorus taken up in the process. Thisphosphorus is then returned back to the process in the form of digestersupernatant. Consequently, this reduces the efficiency of phosphorusremoval in the process and results in higher levels of phosphorus in theeffluent. A partial solution to this problem is to employ a side streamprocess called ‘Phos-Pho Strip’ as described in U.S. Pat. No. 3,654,147.In this process, the activated sludge, which has a high concentration ofphosphorus, passes from the clarifier to a phosphorus stripper. In thestripper, phosphorus is released into the filtrate stream by either:creating anaerobic conditions; adjusting the pH; or extended aeration.The resulting phosphate-rich filtrate stream passes to a chemicalprecipitator. The phosphate-free effluent stream is added to the maineffluent stream, the waste stream from the precipitator containing thephosphates is discarded, and the phosphate-depleted activated sludge isreturned to the main process.

[0007] Another disadvantage with the process in U.S. Pat. No. '883 isthat significant design limitations are imposed by the settlingcharacteristics of the sludge in the clarifier. For example, the processcannot operate at very high process solids levels or high sludgeretention times. As a result, the system is generally considered to beinefficient and there is a high generation rate of waste sludge.

[0008] A second type of biological treatment is referred to as amembrane bioreactor which can be combined with chemical precipitationtechniques. In a simple example, precipitating chemicals are added to anaerobic tank containing or connected to a membrane filter. As above,however, dosages of precipitating chemicals substantially in excess ofthe stoichiometric amount of phosphates are required to achieve lowlevels of phosphates in the effluent. This results in excessive sludgegeneration and the presence of metallic precipitates which increase therate of membrane fouling or force the operator to operate the system atan inefficient low sludge retention time.

[0009] Also relevant to the present invention is U.S. Pat. No. 5,658,458which discloses a treatment for activated sludge involving theseparation of trash and inerts. Generally, the process consists of ascreen which removes relatively large pieces of ‘trash’ and ahydrocyclone which uses a centrifugal force to separate the organicsfrom the inerts. The activated sludge is recycled back to the system andthe trash and inerts are discarded.

SUMMARY OF THE INVENTION

[0010] It is an object of the present invention to remove phosphorousfrom waste water. In some aspects, the invention provides a process fortreating water to remove phosphorous and nitrogen. The process includesthree zones, an anaerobic zone, an anoxic zone and an aerobic zone. Inthe anaerobic zone, an anaerobic mixed liquor has organisms whichrelease phosphorous into the anaerobic mixed liquor and store volatilefatty acids from the anaerobic mixed liquor. In the anoxic zone, ananoxic mixed liquor has organisms which metabolize stored volatile fattyacids, uptake phosphorous and denitrify the anoxic mixed liquor. In theaerobic zone, an aerobic mixed liquor has organisms which metabolizestored volatile fatty acids, uptake phosphorous and nitrify the aerobicmixed liquor.

[0011] Water to be treated flows first into the anaerobic zone to jointhe anaerobic mixed liquor. Anaerobic mixed liquor flows to the anoxiczone to join the anoxic mixed liquor. Anoxic mixed liquor flows bothback to the anaerobic zone to join the anaerobic mixed liquor and to theaerobic zone to join the aerobic mixed liquor. The aerobic mixed liquorflows to the anoxic zone to join the anoxic mixed liquor and alsocontacts the feed side of a membrane filter. The membrane filter treatsthe aerobic mixed liquor to produce a treated effluent lean inphosphorous, nitrogen, BOD, suspended solids and organisms at a permeateside of the membrane filter and a liquid rich in rejected solids andorganisms.

[0012] Some or all of the material rejected by the membrane filter isremoved from the process. This may be done by locating the membranefilter outside of the aerobic zone and directly removing the liquid richin rejected solids and organisms from the retentate or feed side of themembrane filter. Alternatively, the membrane filter may be located inthe aerobic zone so that the material rejected by the membrane filtermixes with the aerobic mixed liquor. The material rejected by themembrane filter is then removed by removing aerobic mixed liquor.Further alternatively, the liquid rich in material rejected by themembrane filter may be recycled to the anoxic or aerobic zones. Thematerial rejected by the membrane filter is then removed by removingaerobic mixed liquor. Combinations of the first and third methodsdescribed above may also be used.

[0013] The steps described above are performed substantiallycontinuously and substantially simultaneously. In the anaerobic zone,fermentive bacteria convert BOD into volatile fatty acids. Bio-Porganisms use the volatile fatty acids as a carbon source. In doing so,they release phosphorus into the liquor, and store volatile fatty acidsfor later use. The stored carbon compounds may come from volatile fattyacids produced in the anaerobic zone or from materials produced externalto the process or both. For example, upstream waste water fermentationcan occur either in prefermentation units specifically designed for thispurpose, or inadvertently in the sewage system. Subsequently, in theanoxic and aerobic zones, the Bio-P organisms metabolize the storedvolatile fatty acids and uptake phosphates from the liquor. The recyclebetween the anoxic and anaerobic zones allows the process to operatesubstantially continuously.

[0014] The stream exiting the aerobic zone passes through the membranefilter. In the membrane filter, phosphorus-rich activated sludge, finelysuspended colloidal phosphorus, bacteria, and other cellular debris arerejected by the membrane. A waste activated sludge containing materialrejected by the membrane filter, optionally combined with aerobic mixedliquor, flows to a sludge management or processing system. A phosphorouslean effluent is produced at the permeate side of the membrane filter.The effluent is also reduced in nitrogen as a result of the anoxic andaerobic zones and the recycle between them.

[0015] The membrane filter removes colloidal phosphorus and bacteriawhich would normally pass through a clarifier. Although the absoluteamount of colloidal solids is relatively small, the percentage ofphosphorus in the colloids is surprisingly high and its removal resultsin unexpected low levels of phosphorus in the effluent. With membranefilters to remove biomass from the effluent stream, a fine biomass canbe maintained in the anaerobic reactor. This may result in enhancedreaction rates and higher than anticipated release of phosphorus in theanaerobic reactor, with resulting higher uptake of phosphorus in theanoxic and aerobic zones. Further, since the process is not limited bythe settling characteristics of the sludge, the process is able tooperate at very high process solid levels, preferably with an MLSSbetween 3 and 30 mg/L and short net hydraulic retention times,preferably between 2 and 12 hours. The short HRT allows increasedthroughput of waste water for a given reactor size. In addition, sincethe design avoids chemical precipitation of phosphates upstream of themembrane filters, there is reduced membrane fouling which furtherenhances the performance of the process. Moreover, contaminants in thesludge resulting from precipitating chemicals are reduced permitting thesystem to operate at a high sludge age. At high sludge retention times,preferably between 10 and 30 days, an unexpected significant crystallinephosphorus accumulation occurs in the biomass, effectively removingphosphorus from the system. As well, there is lower net sludgegeneration.

[0016] The processes described above optionally includes one of two sidestream processes. In a first side stream process, a liquid lean insolids but containing phosphorous is extracted from the anaerobic mixedliquor. Phosphorous is precipitated from that liquid to produce aphosphorous lean liquid which leaves the process as effluent or isreturned to the anoxic or aerobic zones. In a second side streamprocess, anaerobic mixed liquor is removed to a reaction zone andtreated to form a liquid rich in insoluble phosphates. The liquid richin insoluble phosphates is treated in a hydrocyclone to separate outinsoluble phosphates and create a liquid lean in insoluble phosphates.The liquid lean in insoluble phosphates is returned to the anoxic zone.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Embodiments of the present invention will be described below withreference to the following figures:

[0018]FIG. 1 is a schematic diagram illustrating a first process.

[0019]FIG. 2 is a schematic diagram illustrating a second process.

[0020]FIG. 3 is a schematic diagram illustrating a third process.

[0021]FIG. 4 is a schematic diagram illustrating a first side streamprocess.

[0022]FIG. 5 is a schematic illustration of a second side streamprocess.

DETAILED DESCRIPTION OF THE INVENTION

[0023] A first process is shown in FIG. 1. An influent 12, whichcontains BOD, ammonia and phosphates, enters an anaerobic zone 14 to mixwith anaerobic mixed liquor contained there. An anaerobic exit stream 16carries anaerobic mixed liquor from the anaerobic zone 14 to mix withanoxic mixed liquor contained in an anoxic zone 18. An anoxic exitstream 20 carries anoxic mixed liquor from the anoxic zone 18 to mixwith aerobic mixed liquor contained in an aerobic zone 22. An aerobicexit stream 24 carries aerobic mixed liquor from the aerobic zone 22 tothe retentate or feed side a membrane filter 26 located outside of theaerobic zone 22 and preferably having microfiltration or ultrafiltrationmembranes. A waste activated sludge stream 32, consisting of componentsor material rejected by the membrane filter 26, exits from the membranefilter 26. An effluent stream 34 exits from the permeate side of themembrane filter 26 and is substantially phosphate reduced. A denitrifiedliquor recycle 28 carries anoxic mixed liquor from the anoxic zone 18 tomix with the anaerobic mixed liquor in the anaerobic zone 14. As well, anitrified liquor recycle 30 carries aerobic mixed liquor from theaerobic zone 22 to mix with the anoxic mixed liquor in the anoxic zone18.

[0024] In the anaerobic zone 14, fermentive bacteria in the anaerobicmixed liquor convert BOD into volatile fatty acids. Under anaerobicconditions and in the absence of nitrates, Bio-P organisms grow whichuse volatile fatty acids as a carbon source. In doing so, they releasephosphorus into the liquor, and store volatile fatty acids as asubstrate for later use. Anoxic mixed liquor from the denitrified liquorrecycle 28 decreases the concentration of nitrates in the anaerobicmixed liquor in the first anaerobic zone 14, should there be any, whichenhances the selection and growth of Bio-P organisms.

[0025] In the anoxic zone 18, Bio-P organisms in the anoxic mixed liquormetabolize the stored volatile fatty acids, providing energy for growth,and uptake phosphates from the solution. Thus, the efficiency ofphosphate uptake in the anoxic zone 18 is related to the uptake ofvolatile fatty acids in the anaerobic zone 14 particularly given thenitrified liquor recycle 30 from the first aerobic zone 22 to the anoxiczone 18. In the anoxic zone 18, denitrifying bacteria also convertnitrates to N2 gas.

[0026] In the aerobic zone 22, Bio-P organisms in the aerobic mixedliquor further metabolize the stored volatile fatty acids, providingmore energy for growth, and further uptake of phosphates from theaerobic mixed liquor. In addition NH3 is converted to nitrates to berecycled to the anoxic zone 18.

[0027] Aerobic mixed liquor flows to the feed or retentate side of themembrane filter 26. The membrane filter 26 rejects phosphorus-richactivated sludge, finely suspended colloidal phosphorus, bacteria,inorganic particles such as grit, trash and other cellular debris.Liquid containing this rejected material forms a waste activated sludgestream 32 which may either be discarded or sent to a secondary sludgeprocessing system, such as aerobic or anaerobic digestion.Alternatively, some of the liquid containing material rejected by themembranes can be recycled to the aerobic zone 22 through a retentaterecycle stream 29 or to the anoxic zone 18 through a second retentaterecycle stream 29′. When liquid containing material rejected by themembranes is recycled to the anoxic zone 18, its flow and concentrationis included in any calculation of recycle from the aerobic zone 22 tothe anoxic zone 18. In particular, where a second recycle stream 29′ ofhigh flux is used, it may not be necessary to provide either a retentaterecycle stream 29 or a nitrified liquor recycle 30.

[0028] Referring now to FIG. 2, a second process is shown. The secondprocess is similar to the first process but with modifications asdescribed below. The retentate liquid containing material rejected bythe membranes does not leave the process directly but are recycled backto process. This material leaves the process indirectly, combined withsludge from the process in general, as waste sludge taken from theaerobic zone 22. This waste sludge flows out of the process in a secondwaste activated sludge stream 31 from the aerobic zone 22 and may beperiodically discarded or sent to a secondary sludge management orprocessing system, such as aerobic or anaerobic digestion. The liquidcontaining material rejected by the membranes can be recycled to theaerobic zone 22 through a retentate recycle stream 29 or to the anoxiczone 18 through a second retentate recycle stream 29′. When liquidcontaining material rejected by the membranes is recycled to the anoxiczone 18, its flow and concentration is included in any calculation ofrecycle from the aerobic zone 22 to the anoxic zone 18.

[0029] Referring now to FIG. 3, a third process is shown. The thirdprocess is similar to the first process but with modifications asdescribed below. In the third process, a second membrane filter 33 isimmersed in the aerobic zone 22. The second membrane filter 33 is drivenby suction on an interior surface (permeate side) of the membranes, theoutside surface (retentate or feed side) of the membranes is in fluidcommunication with the aerobic mixed liquor. Thus material rejected bythe second membrane filter 33 mixes with the aerobic mixed liquorwithout requiring a retentate recycle stream. As in the second process,material rejected by the membranes leaves the process indirectly,combined with sludge from the process in general, as waste sludge takenfrom the aerobic zone 22. This waste sludge flows out of the process ina second waste activated sludge stream 31 from the aerobic zone 22 andmay be periodically discarded or sent to a secondary sludge managementor processing system, such as aerobic or anaerobic digestion.

[0030] Although there are differences between the processes describedabove, they are similar in many respects and are operated under similarprocess parameters. Net hydraulic retention times (HRT) for all threezones combined (ie. sum of the volume of all three zones divided by thefeed rate) ranges from 1 to 24 hours, is preferably between 2 and 12hours, more preferably between 4.5 and 9 hours. In general, an increasein HRT often increases effluent quality, but increase in effluentquality is less for each additional hour of HRT. Similarly, reducing theHRT increases the output of the process for a given plant size, buteffluent quality decreases more rapidly for each hour less of HRT.Sludge retention time ranges from 5 to 40 days and is preferably between10 and 25 days. MLSS concentration typically ranges between 3 and 30 g/Land is preferably between 5 and 15 g/L. Recycle ratio (recycle to feed)of the nitrified liquor recycle 30 typically ranges between 0.5 and 5and is preferably between 1 and 2 where the process is used primarily toremove phosphorous. Recycle ratio of the nitrified liquor recycle 30typically ranges between 1 and 8 and is preferably between 2 and 4 wherethe process is used primarily to remove phosphorous but nitrogenreduction is also important.

[0031] The recycle ratio (recycle to feed) of the denitrified liquorrecycle 28 typically ranges from 0.5 to 3, preferably between 1 and 2.Less stringent phosphorous effluent requirements may be met efficientlywith a recycle ratio between 0.5 and 1, but typical effluentrequirements require a recycle ratio of over 1. Recycle ratios over 2may result in increased phosphorous removal but only where residualnitrogen levels in the anoxic zone 18 are very low. In typicalprocesses, nitrogen levels in the anaerobic zone 14 become detrimentallyhigh with recycle ratios over 2.

[0032]FIG. 4 shows a first side stream process. Although the first sidestream process is shown in use with the first process of FIG. 1, it mayalso be used with the second and third processes of FIGS. 2 and 3. Ingeneral, a portion of the anaerobic mixed liquor is treated in asolid-liquid separation device 42. A return stream 40 rich in suspendedsolids, including activated sludge and organic impurities, is returnedto the anoxic zone 18 and a solids lean stream (a first phosphate-richsupernatant or filtrate 44) which is rich in phosphorus is fed to acrystallizer or precipitator 50, where insoluble crystalline phosphatesare removed. With this method, phosphorus is removed from the wastewater treatment cycle with near stoichiometric amounts of precipitatingchemicals. Phosphorous removal is enhanced because less phosphorousneeds to be taken up by the Bio-P organisms in the main process. Theability to control phosphorus removal in the crystallizer orprecipitator 50 through pH adjustments helps ensure that adequatephosphorus is available in the process for microbial growth to occur.Finally, a useful by-product, high purity struvite, may be recoveredwhich can be used as a fertilizer. Alternatively, phosphates may beprecipitated as a metal salt.

[0033] In greater detail, a first side stream process is shown generallyat 37 and draws anaerobic mixed liquor from the anaerobic zone 14. Thefirst side stream process 37 removes phosphorous from the main processthereby assisting to reduce the build-up of phosphorous in the system.In the anaerobic zone 14, activated sludge releases phosphorous into theliquor. As such, the anaerobic zone 14 contains liquor with the highestphosphorous concentration. A first phosphate-rich flow stream 38 istaken from the anaerobic zone 14 and sent to a separator 42. Theseparator 42 can use a membrane or other filter media such as a sandfilter, a cloth filter, or fibre braids. The separator 42 can also be aclarifier as the inventors' experience with this process has shown theanaerobic sludge to be surprisingly settleable. A solids rich returnstream 40, comprising the phosphate-depleted sludge and insolubleorganics, exits from the separator 42 and recycles back to the anoxiczone 18. A first phosphate-rich supernatant or filtrate 44 exits theseparator 42, is mixed with precipitating chemicals 46, such as calciumor magnesium, and a combined stream 48 is fed into a crystallizer orprecipitator 50. Since the first phosphate-rich supernatant or filtrate44 is substantially free of organic impurities, the number ofundesirable side reactions with the precipitating chemicals 46 isreduced. As such, the precipitating chemicals 46 can be added in nearstoichiometric amounts to precipitate out the insoluble phosphates.

[0034] A preferred method of crystallization involves using granularseed materials, preferably high density coral sands with grain sizebetween 0.25 and 2.0 mm, to initiate and aid crystallization.Preferably, the addition of magnesium, ammonium and possibly additionalphosphates allow high purity struvite (MgNH3PO4*6H20) to form andcollect at the bottom of the crystallizer or precipitator 50. A bottomsflow stream 54 containing the insoluble phosphates is removed from thesystem and collected. A crystallizer or precipitator exit stream 52,which is both phosphate and nitrate lean may be returned to the anoxiczone 18 (52 a), the aerobic zone 22 (52 b) or be combined with theeffluent stream 34 (52 c) depending on whether it needs furthertreatment. For example, crystallizer or precipitator exit stream 52 highin COD is returned to the aerobic zone to decrease its COD concentrationbefore it is discharged from the process.

[0035] A preferred method of precipitation involves using alum as theprecipitating chemical 46. Surprisingly, there appears to be an optimumdosage ranging between 400 and 800 mg/L at which maximum phosphorous isremoved. Within this range, phosphorous removal is over 50% and may beas high as 93%. Phosphorous removal between 75% to 85% was reliablyachieved in testing using a dosage of 600 mg/L of alum.

[0036]FIG. 5 shows a second side stream process. Although the secondside stream process is shown in use with the first process of FIG. 1, itmay also be used with the second and third processes of FIGS. 2 and 3.In the second side stream process, sludge is optionally filtered througha screen to remove any large objects, hair or other trash that couldinterfere with the other operations in the side stream process.Subsequently, chemicals are added to a reaction zone to create insolublephosphates. The stream is then passed to a hydrocyclone which separatesthe organics from the inorganics, grit, and inerts, which include theinsoluble phosphates. The phosphates are disposed of as inorganic waste,and the phosphorous-depleted activated sludge is recycled back to theanoxic zone.

[0037] The second side stream process is shown generally at 57 to drawanaerobic sludge from the anaerobic zone 14. The second side streamprocess 57 removes phosphates from the main process thereby assisting toreduce the build-up of phosphates in the system. As discussed above, theanaerobic zone 14 contains liquor with the highest phosphateconcentration. A second phosphate-rich flow stream 58 exits from theanaerobic zone 14 and flows to a screen 60 to separate trash etc whichleaves the process to be treated further or discarded. A screen exitstream 62 from the screen 60 is mixed with a precipitating chemicalsflow stream 64, which contains chemicals such as ferrous chloride andaluminum sulphate, and a second combined stream 66 is sent to a reactionzone 70, or alternately to a precipitation tank. A reaction zone exitstream 72 from the reaction zone 70 flows to a hydrocyclone 74. Thehydrocyclone 74 separates the organic material from the inorganics,including insoluble phosphates, grit, and other inerts, due to thedifferences in densities. Hydrocyclone bottoms 76, including theinsoluble phosphates, grit, and other inerts, are landfilled, applied tothe land or otherwise processed or wasted. A sixth waste activatedsludge stream 78 which exits the hydrocyclone 74 is sent back to theanoxic zone 18.

EXAMPLES

[0038] An experimental reactor was set up as shown in FIG. 3. Themembrane filter consisted of four ZEEWEEDTM ZW-10TM modules produced byZenon Environmental Inc. having a total of 40 square feet of membranesurface area. A control reactor was set up as shown in FIG. 2 but (a)using a clarifier instead of the membrane filter 26, (b) recycling theclarifier bottoms to the anoxic zone 18 and (c) not using a retentaterecycle stream 29 or nitrified liquor recycle 30. Both reactors had avolume of 1265 L, the volume of the clarifier not being counted asreactor volume. Sludge retention time (SRT) was kept constant at 25days.

[0039] Three experimental runs were conducted with the experimentalreactor at hydraulic retention times (HRTs) of 9 hours, 6 hours and 4.5hours produced by varying the feed flow rate. The control reactor wasrun successfully at a hydraulic retention time of 9 hours using the sameoperating parameters as for the run of the experimental reactor with a 9hour HRT. Running the control reactor at a hydraulic retention time of 6hours was attempted, but adequate operation could not be achieved(because the clarifier failed), most conventional processes running atan HRT of about 12. The sizes of the zones and the HRTs of each zone aresummarized in Table 1 below. TABLE 1 Process Anaerobic Aerobic OverallZone Sizing Zone Anoxic Zone Zone Bioreactor Volume 1/11 4/11 6/11 11/11Fraction Working 115 460 690 1265 Volume Operating [hr] [hr] [hr] [hr]HRTs Run #1 0.82 3.27 4.91 9.0 Run #2 0.55 2.18 3.27 6.0 Run #3 0.411.64 2.45 4.5

[0040] During the first run, the experimental and control reactors wereoperated at a 9 hour HRT for 16 weeks. The MLSS concentration variedbetween 3-5 g/L during this period. A summary of the average P and Nconcentrations for both reactors is shown in Table 2 below. As shown inthat table, the experimental reactor achieved a greater reduction ofo-PO4. Effluent P was generally below 0.3 mg/L for the experimentalprocess while effluent P for the control process varied from 0.2-0.7mg/L. Both processes had similar reduction of NH3. The experimentalreactor had not been optimized for nitrogen removal. The nitrifiedrecycle was set nominally at a 1:1 (recycle to feed) to be the same asthe control process. Other experiments, to be described below, revealedthat a recycle ratio of 3:1 produced better nitrogen removal in theexperimental process. Nevertheless, the experimental process removedgreater than 80% of total nitrogen at the 1:1 recycle ratio. TABLE 2Anaero- Aero- Para- In- bic Anoxic bic Reduc- Process meter fluent ZoneZone Zone Effluent tion Experi- o-PO₄ 3.04 8.66 2.61 0.17 0.11 96.4%mental [mg/l] NH3-N 22.0 12.7 5.7 0.1 0.09 99.6% [mg/L] NO3-N — 0.130.28 5.68 5.72 [mg/L] Control o-PO₄ 3.04 6.09 5.07 0.27 0.50 83.6%[mg/L] NH3-N 22.0 11.3 5.4 0.04 0.11 99.5% [mg/L] NO3-N — 0.15 0.10 5.792.60 [mg/L]

[0041] During the second run, the experimental reactor was operated at a6 hour HRT for about 14 weeks. The MLSS concentration increased fromabout 4 mg/L at the start to about 8 mg/L at the end of the run. By theend of the run, the experimental process had stabilized in terns of VFAuptake and phosphorous release in the anaerobic section. There was aslow and steady improvement in performance as the experimental runprogressed, the monthly average effluent P dropping from 0.178 mg/L to0.144 mg/L to 0.085 mg/L over the approximately three months of thetest. The inventors believe that at least part of this improvement canbe attributed to the increase in MLSS over the duration of the test.Effluent NH3 was less than 0.5 mg/L and total nitrogen removal wasgreater than 80%.

[0042] As mentioned above, the control reactor could not be operateadequately at this HRT. During periods when the control reactor wasoperated, effluent P varied between 0.1-0.9 mg/L.

[0043] During the third run, the experimental reactor only was run at anHRT of 4.5 hours. MLSS concentration increased to 15 g/L. Effluent Pconcentrations were generally below 0.5 mg/L over a three month period,still better than the P removal of the control reactor operated at a 9hour HRT.

[0044] In other experiments, the experimental process was operated at anHRT of 6 hours but the recycle from the aerobic zone to the anoxic zonewas modified from a recycle ratio (recycle to feed) of 1:1 to 3:1. N andP removal were measured at each recycle ratio and the results includedin Table 3 below. Nitrogen removal increased with the increased recycleratios while P removal was generally unaffected. TABLE 3 InfluentEffluent Effluent Total N Total N PO₄ Recycle (mg/L as (mg/L asN-Removal (mg/L as P-Removal Ratio N) N) Efficiency P) Efficiency 1:149.6 11.8 76.4% 0.07 98.9% 1.5:1 49.8 9.0 81.6% 0.15 97.7% 2:1 40.7 5.885.8% 0.15 97.3% 3:1 42.1 5.3 87.5% 0.12 97.8%

[0045] It is to be understood that what has been described are preferredembodiments of the invention. The invention nonetheless is susceptibleto certain changes and alternative embodiments fully comprehended by thespirit of the invention as defined by the claims below.

We claim:
 1. A process for treating water to remove phosphorous andnitrogen comprising the steps of: (a) providing an anaerobic zone havingan anaerobic mixed liquor having organisms which release phosphorousinto the anaerobic mixed liquor and store volatile fatty acids from theanaerobic mixed liquor; (b) providing an anoxic zone having an anoxicmixed liquor having organisms which metabolize stored volatile fattyacids, uptake phosphorous and denitrify the anoxic mixed liquor; (c)providing an aerobic zone having an aerobic mixed liquor havingorganisms which metabolize stored volatile fatty acids, uptakephosphorous and nitrify the aerobic mixed liquor; (d) flowing water tobe treated into the anaerobic zone; (e) flowing anaerobic mixed liquorto the anoxic zone; (f) flowing anoxic mixed liquor to the anaerobiczone; (g) flowing anoxic mixed liquor to the aerobic zone; (h) flowingaerobic mixed liquor to the anoxic zone; (i) contacting aerobic mixedliquor against the feed side of a membrane filter; (j) producing atreated effluent lean in phosphorous, nitrogen, BOD OR COD, suspendedsolids and organisms from a permeate side of the membrane filter; and,(g) removing some or all of the material rejected by the membrane filterfrom the process, wherein the steps above are performed substantiallycontinuously and substantially simultaneously and wherein the MLSS isbetween 3 and 30 g/L.
 2. The process of claim 1 wherein the MLSS isbetween 5 and 15 g/L.
 3. The process of claim 1 wherein materialrejected by the membrane filter is also mixed with the aerobic mixedliquor.
 4. The process of claim 3 wherein the step of removing materialrejected by the membrane filter from the process is accomplished byremoving aerobic mixed liquor containing material rejected by themembrane filter.
 5. The process of claim 1 wherein material rejected bythe membrane filter is also mixed with the anoxic mixed liquor.
 6. Theprocess of claim 1 wherein the net hydraulic retention time for theanaerobic, anoxic and aerobic zones combined is between 2 and 12 hours.7. The process of claim 1 wherein the net hydraulic retention time forthe anaerobic, anoxic and aerobic zones combined is between 2 and 9hours.
 8. The process of claim 1 wherein the net hydraulic retentiontime for the anaerobic, anoxic and aerobic zones combined is between 4.5and 9 hours.
 9. The process of claim 1 wherein the sludge retention timeis between 10 and 30 days and a crystalline phosphorous accumulationoccurs in the mixed liquor.
 10. The process of claim 1 wherein therecycle ratio between the aerobic zone and the anoxic zone is between 2and
 4. 11. A process for treating water to remove phosphorous andnitrogen comprising the steps of: (a) providing an anaerobic zone havingan anaerobic mixed liquor having organisms which release phosphorousinto the anaerobic mixed liquor and store volatile fatty acids from theanaerobic mixed liquor; (b) providing an anoxic zone having an anoxicmixed liquor having organisms which metabolize stored volatile fattyacids, uptake phosphorous and denitrify the anoxic mixed liquor; (c)providing an aerobic zone having an aerobic mixed liquor havingorganisms which metabolize stored volatile fatty acids, uptakephosphorous and nitrify the aerobic mixed liquor; (d) flowing water tobe treated into the anaerobic zone; (e) flowing anaerobic mixed liquorto the anoxic zone; (f) flowing anoxic mixed liquor to the anaerobiczone; (g) flowing anoxic mixed liquor to the aerobic zone; (h) flowingaerobic mixed liquor to the anoxic zone; (i) contacting aerobic mixedliquor against the feed side of a membrane filter; (j) producing atreated effluent lean in phosphorous, nitrogen, BOD OR COD, suspendedsolids and organisms from a permeate side of the membrane filter; and,(g) removing some or all of the material rejected by the membrane filterfrom the process, (h) extracting a liquid containing phosphorous butlean in solids from the anaerobic mixed liquor; (i) precipitatingphosphorous from the liquid containing phosphorous but lean in solids;and, (j) producing a phosphorous lean effluent from the liquidcontaining phosphorous but lean in solids, wherein the steps above areperformed substantially continuously and substantially simultaneously.12. The process of claim 11 wherein material rejected by the membranefilter is also mixed with the aerobic mixed liquor.
 13. The process ofclaim 12 wherein the step of removing material rejected by the membranefilter from the process is accomplished by removing aerobic mixed liquorcontaining material rejected by the membrane filter.
 14. The process ofclaim 11 wherein material rejected by the membrane filter is also mixedwith the anoxic mixed liquor.
 15. The process of claim 11 wherein thephosphorous is precipitated by the addition of between 400 to 800 mg/Lof alum.
 16. A process for treating water to remove phosphorous andnitrogen comprising the steps of: (a) providing an anaerobic zone havingan anaerobic mixed liquor having organisms which release phosphorousinto the anaerobic mixed liquor and store volatile fatty acids from theanaerobic mixed liquor; (b) providing an anoxic zone having an anoxicmixed liquor having organisms which metabolize stored volatile fattyacids, uptake phosphorous and denitrify the anoxic mixed liquor; (c)providing an aerobic zone having an aerobic mixed liquor havingorganisms which metabolize stored volatile fatty acids, uptakephosphorous and nitrify the aerobic mixed liquor; (d) flowing water tobe treated into the anaerobic zone; (e) flowing anaerobic mixed liquorto the anoxic zone; (f) flowing anoxic mixed liquor to the anaerobiczone; (g) flowing anoxic mixed liquor to the aerobic zone; (h) flowingaerobic mixed liquor to the anoxic zone; (i) treating aerobic mixedliquor in a solid-liquid separator to produce a treated effluent lean inphosphorous, nitrogen suspended solids and organisms and a liquid richin solids rejected by the solid liquid separator; (j) removing some orall of the liquid rich in solids rejected by the solid-liquid separator;(k) extracting a phosphorous containing permeate lean in solids from theanaerobic mixed liquor; (l) precipitating phosphorous from thephosphorous containing permeate lean in solids; and, (m) producing aphosphorous lean effluent from the phosphorous containing permeate leanin solids, wherein the steps above are performed substantiallycontinuously and substantially simultaneously.
 17. The process of claim16 wherein the phosphorous is precipitated by the addition of between400 to 800 mg/L of alum.